Infrared detecting circuit and an infrared detector

ABSTRACT

An infrared detecting circuit is provided with a current-to-voltage converting circuit including a capacitor connected with an inverting input terminal and an output terminal of an operational amplifier and a resistance circuit element connected in parallel with the capacitor, an inverting amplifying circuit connected with an output side of the current-to-voltage converting circuit, a band-pass filter circuit connected with an output side of the voltage amplifying circuit, and an output circuit connected with an output side of the band-pass filter circuit. The infrared detecting circuit and an infrared detector including this circuit can be miniaturized.

TECHNICAL FIELD

This invention relates to an infrared detecting circuit and an infrareddetector.

BACKGROUND ART

An infrared detector of the prior art is, as shown in FIG. 15, providedwith a pyroelectric element 100 for detecting infrared rays radiatedfrom a human body, a current-to-voltage converting circuit 200 forconverting a detection current signal of the pyroelectric element 100into a voltage signal, a coupling capacitor C30 connected with thecurrent-to-voltage converting circuit 200, a voltage amplifying circuit300 connected with an output of the coupling capacitor C30, a low-passfilter 400 connected with the voltage amplifying circuit 300, ahigh-pass filter 500 connected with the low-pass filter 400, anamplifying circuit 600 connected with the high-pass filter 500, and anoutput circuit 700 connected with the amplifying circuit 600.

The current-to-voltage converting circuit 200 includes a FET (FieldEffect Transistor) having a gate connected with the pyroelectric element100, a resistor Rg connected in parallel with the opposite ends of thepyroelectric element 100, and a resistor Rs provided between a source ofthe FET and a ground. The low-pass filter 400 and the high-pass filter500 each include a switched capacitor.

The infrared detector thus constructed operates as follows. A detectioncurrent signal outputted from the pyroelectric element 100 is convertedinto a voltage signal by the resistor Rg and applied to the gate of theFET, whereby a drain current flows from the source of the FET to a drainthereof. A source voltage is generated between the FET and the resistorRs by the flow of the drain current. After direct-current (dc)components of the source voltage is cut by the coupling capacitor C30,and the source voltage is amplified at an amplification factor(1+R20/R10) by the voltage amplifying circuit 300, and is consequentlyhandled as a voltage signal in a specified frequency band whosehigh-frequency components and low-frequency components have been cut bythe low-pass filter 400 and the high-pass filter 500. This voltagesignal is amplified at a set gain by the amplifying circuit 600, and isoutputted as a detection signal from the output circuit 700 after beingcompared with a specified level therein.

However, the above coupling capacitor C30 needs to have a large capacityin order to pass frequency components around 1 Hz expressing the motionof the human body, etc. Thus, a large-size capacitor has to be used assuch. The large-size capacitor is obliged to be separately externallyprovided since it is difficult to integrate. Such an externally providedcoupling capacitor C30 has been standing as a large hindrance tominiaturization or integration of the infrared detector.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide an infrareddetecting circuit and device which are free from the problems residingin the prior art.

According to an aspect of the present invention, an infrared detectingcircuit is constructed by a current-to-voltage converting circuit, anamplifying circuit for amplifying the voltage signal outputted from thecurrent-to-voltage converting circuit, a band-pass filter circuitincluding a switched capacitor and adapted to pass components of avoltage signal from the amplifying circuit in a specified frequencyband, a clock generating circuit for generating a reference clock signalfor controlling the switched capacitor, and an output circuit foroutputting a voltage signal outputted from the band-pass filter circuitas a detection signal when the voltage signal is at a threshold level orhigher.

The current-to-voltage converting circuit is to be connected with apyroelectric element operable to generate a current signal in accordancewith a received infrared ray to thereby convert the current signaloutputted from the pyroelectric element into a voltage signal. Thecurrent-to-voltage converting circuit includes an operational amplifierconnected with the pyroelectric element, a capacitor, and a feedbackcircuit for feeding back a direct current component. The capacitor andthe feedback circuit are connected between an output terminal and aninverting input terminal of the operational amplifier in parallel witheach other.

A pyroelectric element is connected with the current-to-voltageconverting circuit of the above-mentioned infrared detecting circuit toproduce an infrared detector.

These and other objects, features and advantages of the presentinvention will become more apparent upon a reading of the followingdetailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of an infrared detector accordingto an embodiment of the present invention;

FIG. 2 is a circuit diagram of an infrared detecting circuit used in theinfrared detector;

FIG. 3 is a circuit diagram of a first modified infrared detectingcircuit used in the infrared detector;

FIG. 4 is a circuit diagram of a second modified infrared detectingcircuit used in the infrared detector;

FIG. 5 is a circuit diagram of a third modified infrared detectingcircuit used in the infrared detector;

FIG. 6 is a circuit diagram of a fourth modified infrared detectingcircuit used in the infrared detector;

FIG. 7 is a circuit diagram of a fifth modified infrared detectingcircuit used in the infrared detector;

FIG. 8 is a circuit diagram of a main portion of a sixth modifiedinfrared detecting circuit used in the infrared detector, showing acurrent-to-voltage converting circuit and a voltage amplifying circuit;

FIG. 9 is a circuit diagram of a seventh modified infrared detectingcircuit used in the infrared detector;

FIG. 10 is a circuit diagram of an eighth modified infrared detectingcircuit used in the infrared detector;

FIG. 11 is a circuit diagram of a main portion of a ninth modifiedinfrared detecting circuit used in the infrared detector, showing acurrent-to-voltage converting circuit and a voltage amplifying circuit;

FIG. 12 is a circuit diagram of a main portion of a tenth modifiedinfrared detecting circuit used in the infrared detector, showing acurrent-to-voltage converting circuit and a voltage amplifying circuit;

FIG. 13 is a circuit diagram of a main portion of an eleventh modifiedinfrared detecting circuit used in the infrared detector, showing acurrent-to-voltage converting circuit and a voltage amplifying circuit;

FIG. 14 is a circuit diagram of a switch controlling portion of theinfrared detecting circuit shown in FIG. 13; and

FIG. 15 is a circuit diagram showing a prior art infrared detectingcircuit.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1 explodedly showing a construction of an infrareddetector according to an embodiment of the present invention, theinfrared detector comprises a disk-shaped base 11 having three leadwires 12 connected with the bottom surface thereof, a disk-shapedprinted circuit board 16 mounted on the upper surface of the base 11 viatwo furrings 17, a pyroelectric element 1 mounted substantially in themiddle of the printed circuit board 16 and having a rectangular lightdetecting surface, a bottomed hollow cylindrical can 13 having anoptical filter window 14 in its ceiling wall and adapted to cover theprinted circuit board 16, and a dome-shaped large-diameter condenserlens 15 mounted on the ceiling wall of the can 13. The condenser lens 15is a multi-lens including a number of lens.

Infrared rays radiated from a human body and passed through the lens 15and the optical filter window 14 are incident on the pyroelectricelement 1. An infrared detecting circuit for processing a detectioncurrent signal is mounted on the rear surface of the printed circuitboard 16. The infrared detecting circuit is integrated into one piece.

A detailed construction of the infrared detecting circuit is shown inFIG. 2. This infrared detecting circuit is provided with acurrent-to-voltage converting circuit 2 for converting a current signalfrom the pyroelectric element 1 into a voltage signal, a voltageamplifying circuit 3 connected with an output of the current-to-voltageconverting circuit 2, a band-pass filter circuit 4 connected with anoutput of the voltage amplifying circuit 3, and an output circuit 5connected with an output of the band-pass filter circuit 4.

The pyroelectric element 1 generates polarization charges correspondingto a temperature increase when temperature increases due to radiatedheat waves, and outputs these polarization charges as a detectioncurrent signal.

The current-to-voltage converting circuit 2 includes an operationalamplifier 21 having an inverting input terminal connected with one endof the pyroelectric element 1, a feedback capacitor Cf connected betweenan output terminal and the inverting input terminal of the operationalamplifier 21, and a resistance circuit element Z connected in parallelwith the feedback capacitor Cf. A power supply E for outputting areference voltage Vr used to set an operating point of the voltagesignal to be outputted is connected between the non-inverting inputterminal of the operational amplifier 21 and a ground. Since thecurrent-to-voltage converting circuit 2 includes the operationalamplifier 21 and gives a negative feedback via the resistance circuitelement Z, the variation of the operating point is suppressed. Thiseliminates the need for a coupling capacitor conventionally used to cutthe variation of the operating point, consequently making it possible toreduce the size of the infrared detecting circuit greatly.

The current-to-voltage converting circuit 2 is provided with thefeedback capacitor Cf. The current components in the frequency band ofaround 0.1 to 1.0 Hz of the current outputted from the pyroelectricelement 1 that is important to detect a human body is converted into thevoltage signal by the use of the feedback capacitor Cf. In theconventional current-to-voltage converting circuit which converts acurrent signal from a pyroelectric element into a voltage signal by useof a resistance member, there is the problem that the converted voltagesignal contains a considerable number of noises due to the fact that theresistance member is likely to generate thermal noises. On the other, acapacitor is theoretically seen not to generate thermal noises.Accordingly, it will be apparent that the current-to-voltage convertingcircuit 2 of this embodiment using the capacitor Cf can convert acurrent signal from a pyroelectric element into a voltage signal havingfew noise.

The voltage amplifying circuit 3 is in the form of an invertingamplifying circuit, and includes an operational amplifier 31 having aninverting input terminal connected with an output terminal of thecurrent-to-voltage converting circuit 2 via a resistor R1, and aresistor R2 provided between an output terminal and the inverting inputterminal of the operational amplifier 31. Further, the power supply Efor outputting the reference voltage Vr used to set the operating pointof the voltage signal to be outputted is connected with a non-invertinginput terminal of the operational amplifier 31.

Since the operational amplifier 21 has a very low output impedance, itis not necessary to consider the input impedance of a circuit connectedwith the output terminal of the operational amplifier 21. Accordingly,the inverting amplifying circuit 3 having a low input impedance isconnected with the output terminal of the operational amplifier 21 tothereby amplify the voltage.

An inverting amplifying circuit is used as the voltage amplifyingcircuit 3 in this embodiment. If a non-inverting amplifying circuit is,conversely, used, the power supply needs to be connected between a gainresistor connected with the inverting input terminal of the operationalamplifier 31 and the ground. If the power supply is connected in thisway, a current flows thereinto and the power supply becomes unstable dueto a voltage decrease caused by the internal resistance therein. Thus, apower supply needs to be connected separately from the power supply Econnected with the pyroelectric element 1 and the current-to-voltageconverting circuit 2. On the other hand, if the voltage amplifyingcircuit 3 is constructed by the inverting amplifying circuit as in thisembodiment, the power supply E can be directly connected with thenon-inverting input terminal of the operational amplifier 31. Further,since the non-inverting input terminal has a high input impedance, nocurrent flows into the power supply E and no voltage decrease occurs dueto the internal resistance in the power supply E. Thus, the referencevoltage of the inverting amplifying circuit can be stabilized.Therefore, the power supply E can be commonly used for thecurrent-to-voltage converting circuit 2 and the voltage amplifyingcircuit 3, and the detecting circuit can be accordingly made smaller.Furthermore, the common use of the power supply E permits the referencevoltage for the current-to-voltage converting circuit 2 and thereference voltage for the voltage amplifying circuit 3 to be identicalwith each other, consequently making the respective operating points ofthe both circuits 2 and 3 identical with each other. Accordingly, theoperating point can be regulated to a negligible variation even withoutproviding a coupling capacitor between the both circuits 2 and 3.

The band-pass filter circuit 4 is provided with a switched capacitorfilter to miniaturize the infrared detecting circuit, and gives aspecified gain to a voltage signal in the frequency band of 0.1 to 1.0Hz, which is important in detecting a human body, and then outputs thevoltage signal. The switched capacitor filter has a resistor section.The resistor section is constructed by a switched capacitor including acapacitor and a switching element such as a MOSFET connected with aclock generating circuit 6. The switching element is turned on and offby a reference clock signal from the clock generating circuit 6 tocharge and discharge the capacitor, whereby causing the capacitor toequivalently function as a resistance member. An equivalent resistancevalue R by the switched capacitor is expressed by R=1/f·C wherein f andC denote a frequency (sampling frequency) of the clock signal given tothe switching element and the capacity of the capacitor, respectively.

In the band-pass filter circuit 4 provided with the above-mentionedswitched capacitor filter, there is a likelihood that a return noiseoccurs when an inputted voltage signal contains many high-frequencycomponents. However, in this embodiment, the current-to-voltageconverting circuit 2 performs the current-to-voltage conversion by thecapacitor Cf. The impedance of the capacitor Cf is expressed by1/(2π·f·Cf). The impedance of the capacitor Cf becomes smaller as thefrequency becomes higher. Accordingly, the voltage signal is outputtedfrom the current-to-voltage converting circuit 2 after thehigh-frequency components are greatly cut. In other words, the voltagesignal having negligible high-frequency components is input to theband-pass filter circuit 4. The return noise in the band-pass filtercircuit 4 can be suppressed.

The output circuit 5 includes a comparator to compare the voltage signaloutputted from the band-pass filter circuit 4 with a specified thresholdlevel, and outputs a detection signal when the voltage signal is at thethreshold level or higher.

This infrared detector operates as follows. A detection current signaloutputted from the pyroelectric element 1 is inputted to thecurrent-to-voltage converting circuit 2. The voltage signal of thedetection current signal that is in the frequency band of 0.1 to 1.0 Hz,which is important in detecting a human body, is converted into avoltage signal by the impedance component 1/(2π·f·Cf) of the capacitorCf. In this way, the high-frequency components are cut and the S/N ratiois improved. Subsequently, after being amplified at an amplificationfactor of R2/R1 in the voltage amplifying circuit 3, the voltage signalconverted by the current-to-voltage converting circuit 2 has componentsin the frequency band of 0.1 to 1.0 Hz cut off by the band-pass filtercircuit 4. Since the high-frequency components are cut in thecurrent-to-voltage converting circuit 2, the occurrence of return noisein the band-pass filter circuit 4 is suppressed. Subsequently, theresulting voltage signal is compared with the threshold level in theoutput circuit 5, which then outputs a detection signal.

As described above, according to the infrared detecting circuit, thecoupling capacitor having been used in the conventional infrareddetecting circuit can be omitted and the power supply E connected withpyroelectric element 1, the current-to-voltage converting circuit 2 andthe voltage amplifying circuit 3 can be commonly used. Thus, theinfrared detecting circuit can be made smaller. Further, since thehigh-frequency components are cut by the current-to-voltage convertingcircuit 2, the occurrence of return noise in the band-pass filtercircuit 4 can be suppressed.

FIG. 3 is a circuit diagram of a first modified infrared detectingcircuit used in the infrared detector. This infrared detecting circuitis identical to the infrared detecting circuit shown in FIG. 2 exceptfor that a second amplifying circuit 7 is additionally provided betweena band-pass filter circuit 4 and an output circuit 5. The secondamplifying circuit 7 is a non-inverting amplifying circuit, and includesan operational amplifier 61 having a non-inverting input terminalconnected with an output terminal of the band-pass filter circuit 4, aresistor R3 connected between an output terminal and an inverting inputterminal of the operational amplifier 61 and a resistor R4 connectedwith the inverting input terminal of the operational amplifier 61. Apower supply E for setting the operating point at a specified level isconnected between the resistor R4 and the ground.

A signal component in the frequency band of 0.1 to 1.0 Hz is importantin detecting human bodies and living organisms. For this reason, thefrequency performance characteristic of a band-pass filter circuit 4arranged prior to the second amplifying circuit 7 is set at a peakfrequency of around 1.0 Hz. Consequently, a signal component in thefrequency of around 0.1 Hz is attenuated by the band-pass filter circuit4. Accordingly, the threshold level of the output circuit 5 has to beset lower so as to detect the attenuated signal component in thefrequency of around 0.1 Hz conform to the attenuated voltage signal,which inevitably makes the influence of noise and the like no negligibleand leads to a higher possibility of an erroneous detection. If it isassumed that a voltage signal in the frequency band around 0.1 Hz isattenuated, for example, by about 20 dB by the band-pass filter circuit4 (the amplitude of the outputted signal becomes 1/10 with respect tothe inputted signal), the amplitude of the voltage signal outputted fromthe voltage amplifying circuit 3 has to be 10 times the threshold levelin order to cause the output circuit 5 to precisely compare the voltagesignal and the threshold level and output the detection signal. In orderto secure an output voltage having an amplitude which is 10 times thethreshold level, a supply voltage level of the operational amplifier 31needs to be at least 10 times the threshold level. The supply voltagelevel is normally about 15 V although it depends on the characteristicsof the operational amplifier 31 used and, accordingly, there is aspecific limit in increasing the supply voltage level of the operationalamplifier 31. On the other hand, it may be considered to maintain thesupply voltage level of the operational amplifier 31 low by reducing thethreshold level of the output circuit 5. However, if the threshold levelis reduced, the output circuit 5 may output the detection signal even inresponse to a noise signal having a small amplitude and, accordingly,there is also a specific limit in reducing the threshold level. Thus,the threshold level is normally set at about half the supply voltagelevel of the operational amplifier 31. However, in such a case, theoutput circuit 5 cannot output the detection signal if the amplitude ofthe voltage signal in the frequency band of around 0.1 Hz is attenuatedto ½ or lower by the band-pass filter circuit 4.

Accordingly, in the current-to-voltage converting circuit of thismodification, the amplifying circuit 7 is connected between theband-pass filter circuit 4 and the output circuit 5 to solve the aboveproblem by amplifying the amplitude level of the voltage signalattenuated in the band-pass filter circuit 4 substantially to thethreshold level.

As described above, according to the first modified infrared detectingcircuit, an erroneous detection by the output circuit 5 is prevented byconnecting the amplifying circuit 7 between the band-pass filter circuit4 and the output circuit 5 to amplify the attenuated amplitude of thevoltage signal outputted from the band-pass filter circuit 4substantially to the threshold level. Accordingly, the detectionaccuracy of this circuit is considerably increased.

FIG. 4 is a circuit diagram of a second modified infrared detectingcircuit. The second modified infrared detecting circuit is identical tothe infrared detecting circuit shown in FIG. 2 except for that ahigh-pass filter 8 is provided between a band-pass filter circuit 4 andan output circuit 5. The high-pass filter 8 includes an operationalamplifier 71 having a non-inverting input terminal connected with anoutput of the band-pass filter circuit 4 via a capacitor C1, a capacitorC2 connected between an output terminal and an inverting input terminalof the operational amplifier 71, and a switched capacitor SC connectedin parallel with the capacitor C2. The switched capacitor SC is adoptedfor the miniaturization of the circuit as described above. A powersupply E is connected with both a non-inverting input terminal of theoperational amplifier 71 and the switched capacitor SC. Since a gain ina passage band of the high-pass filter 8 is expressed by a capacityratio C1/C2, a desired gain can be obtained by suitably setting thecapacities of the capacitors C1 and C2.

The effectiveness of the provision of the amplifying circuit 7 betweenthe band-pass filter circuit 4 and the output circuit 5 is described inthe first modification. If the amplification factor (gain) of theamplifying circuit 7 is set larger, there is a possibility of largelyvarying the operating point of the outputted voltage signal because theoffset component of the voltage signal outputted from the band-passfilter circuit 4 is amplified at the larger amplification factor.

Accordingly, the variation of the operating point is suppressed byconnecting the high-pass filter 8 having a gain between the band-passfilter circuit 4 and the output circuit 5 to cut low-frequencycomponents of the voltage signal outputted from the band-pass filtercircuit 4. Since the voltage signal of frequency components of 0.1 to1.0 Hz is outputted after being amplified to a level approximate to thethreshold level, an erroneous detection by the output circuit 5 can beprevented. Accordingly, the detection accuracy of the detecting circuitis remarkably increased.

As described above, according to the second modified infrared detectingcircuit, an erroneous detection by the output circuit 5 can be preventedsince the variation of the operating point of the voltage signaloutputted from the high-pass filter 8 is suppressed and the frequencycomponents of 0.1 to 1.0 Hz are outputted after being amplified to thelevel approximate to the threshold level. Accordingly, the detectionaccuracy of the detecting circuit is remarkably increased. Further,since the switched capacitor SC is used as a resistor section of thehigh-pass filter 8, the detecting circuit can be miniaturized and thetemperature characteristic thereof can be stabilized.

FIG. 5 is a circuit diagram of a third modified infrared detectingcircuit. The third modified infrared detecting circuit is identical tothe infrared detecting circuit except for that there is provided aband-pass filter circuit 41 constructed by alternately connectinghigh-pass filters and low-pass filters at one stage after another. Thehigh-pass filters and low-pass filters are both constructed by switchedcapacitors. More specifically, a total of five filters of high-passfilter (HPF) 411, low-pass filter (LPF) 412, high-pass filter (HPF) 413,low-pass filter (LPF) 414 and high-pass filter (HPF) 415 aresuccessively connected from an output of a voltage amplifying circuit 3.

These filters not only serves as a band-pass filter circuit forextracting the signal components necessary to detect a human body, butalso have the following functions. The high-pass filter 411 at the firststage suppresses the variation of an operating point by cuttinglow-frequency components contained in a voltage signal outputted fromthe voltage amplifying circuit 3. The low-pass filter 412 at the secondstage outputs a voltage signal in the frequency band of 1 Hz or lowerwhile giving a specified gain thereto.

The high-pass filters 413, 415 at the third and fifth stages suppressthe variation of an operating point by cutting low-frequency componentsof the voltage signals outputted from the low-pass filters 412, 414 atthe second and fourth stages. The capacity of a capacitor used in aswitched capacitor needs to be reduced in order to increase a resistancevalue of a resistor forming the band-pass filter circuit 4 and integratethis circuit 41. If the capacity of the capacitor is reduced,feedthrough noise occurring during a switching operation by a pulsesignal noticeably increases. The increase in the feedthrough noise inturn increases offset components of an operational amplifier, causingthe operating points of the voltage signals outputted from the low-passfilters 412, 414 to largely vary. For this reason, the variations of theoperating points are suppressed by cutting the low-frequency componentsby the high-pass filters 413, 415 connected at the third and fifthstages.

If the band-pass filter circuit 41 is constructed by a single low-passfilter which is then given a large gain (i.e., the band-pass filtercircuit 41 is constructed by the high-pass filter 411, the low-passfilter 412 and high-pass filter 413), there is a possibility that directcurrent components are largely amplified and the voltage signaloutputted from the low-pass filter 412 is saturated. Accordingly, in theband-pass filter circuit 41, the gain is distributed between therespective low-pass filters 412 and 414 to prevent the saturation of thevoltage signals outputted from the low-pass filters 412, 414, and thevariations of the operating points caused by the low-pass filters 412,414 at the second and fourth stages are securely cut by the high-passfilters 413, 415 at the third and fifth stages to remarkably suppressthe variation of the operating point. Further, if the gain isdistributed between the respective low-pass filters 412 and 414, theswitched capacitors of the low-pass filters 412, 414 can be constructedby low-capacity capacitors, enabling the miniaturization of the infrareddetecting circuit.

As described above, according to the third modified infrared detectingcircuit wherein two low-pass filters 412, 414 are provided to distributethe gain between the respective low-pass filters 412 and 414, it can beaccomplished to prevent the saturation of the voltage signals outputtedfrom the low-pass filters 412, 414, and give the high gain to theoutputted voltage signal. Accordingly, the voltage signal outputted fromthe band-pass filter circuit 41 can be sent to the output circuit 5without the necessity of amplifying the voltage signal. In other words,an amplifying circuit need not to be provided, which leads to theminiaturization of the infrared detecting circuit. Further, since thelow-pass filter 412 (414) is arranged between the high-pass filters 411,413, 415, the variation of the operating point resulting from thefeedthrough noise can be assuredly suppressed.

FIG. 6 is a circuit diagram of a fourth modified infrared detectingcircuit. The fourth modified infrared detecting circuit is identical tothe infrared detecting circuit shown in FIG. 2 except for that there isprovided a band-pass filter circuit 42 constructed by connecting alow-pass filter 421 at the first stage and a high-pass filter 422 at thesecond stage. As described in connection with the infrared detectingcircuit shown in FIG. 2, the current-to-voltage converting circuit 2performs the current-to-voltage conversion using the capacitor Cf todecrease the return noise from occurring in the band-pass filter circuit4. However, it is difficult to completely suppress the occurrence of thereturn noise only by applying such a conversion. Accordingly, in thefourth modification, the low-pass filter 421 is provided at the firststage of the band-pass filter circuit 42 to introduce a voltage signalto the high-pass filter 422 after cutting high-frequency components ofthe voltage signal, whereby the occurrence of the return noise resultingfrom the switched capacitor is suppressed. Further, the variation of theoperating point is suppressed since being cut by the high-pass filter422 connected at the second stage. Accordingly, the fourth modifiedinfrared detecting circuit can suppress the occurrence of return noisemore securely.

FIG. 7 is a circuit diagram of a fifth modified infrared detectingcircuit. The fifth modified infrared detecting circuit is identical tothe infrared detecting circuit shown in FIG. 2 except for that there isprovided a current-to-voltage converting circuit 22. Specifically, thecurrent-to-voltage converting circuit 22 is constructed by connecting aresistor Ri and a direct current feedback circuit DF in parallel with anoperational amplifier 221 having an inverting input terminal thereofconnected with a pyroelectric element 1, and with a capacitor Cfconnected between an output terminal and an inverting input terminal ofthe operational amplifier 221. The direct current feedback circuit DF isan integrating circuit, and includes an operational amplifier 223 havinga non-inverting input terminal thereof connected with the outputterminal of the operational amplifier 221, a capacitor C3 connectedbetween the non-inverting input terminal and an output terminal of theoperational amplifier 223, and a resistor R5 connected with thecapacitor C3. Further, a power supply E for setting the operating pointat a specified level is connected between the resistor R5 and a ground.In the current-to-voltage converting circuit 22, the output from theoperational amplifier 221 is fed back by the use of the direct currentfeedback circuit DF. Accordingly, the alternating current components isfed back while being attenuated. The operating point of the outputtedvoltage signal can be more stabilized. Thus, the operating point can bestabilized even without the use of a coupling capacitor.

As described above, the fifth modified infrared detecting circuit, inwhich the direct current feedback circuit is connected with thecurrent-to-voltage converting circuit 22, can reduce the variation ofthe operating point of the outputted voltage signal, enabling a stablecurrent-to-voltage conversion. FIG. 8 is a circuit diagram of a part ofa sixth modified infrared detecting circuit. The sixth modified infrareddetecting circuit is identical to the fifth modification except for theof two reference voltage circuits 1003 and 1004. In the sixth modifiedinfrared detecting circuit, the two reference voltage circuits 1003 and1004 are used to apply a reference voltage for setting the operatingpoints of the pyroelectric element 1, a current-to-voltage convertingcircuit 22 and a voltage amplifying circuit 3. The reference voltagecircuit 1003 is connected with one end of the pyroelectric element 1,whereas the reference voltage circuit 1004 is connected with anon-inverting input terminal of an operational amplifier 221 and anon-inverting input terminal of an operational amplifier 31.

Since the pyroelectric element 1 is externally mounted on the infrareddetecting circuit formed into an integrated circuit, noise may enterthrough a contact between the pyroelectric element 1 and thecurrent-to-voltage converting circuit 2. If only one reference voltagecircuit is provided for applying the reference voltage to thepyroelectric element 1, the current-to-voltage converting circuit 2 andthe voltage amplifying circuit 3, the operations of thecurrent-to-voltage converting circuit 2 and the voltage amplifyingcircuit 3 become unstable due to the influence of noise entered throughthe contact. Thus, the separate reference voltage circuits are providedfor applying the reference voltage to the pyroelectric element 1 and forapplying it to the current-to-voltage converting circuit 22 and thevoltage amplifying circuit 3 in the sixth modification.

However, this construction may have the following problems. If Vn1, Vn2denote a noise voltage normally outputted from the reference voltagecircuit 1003 and a noise voltage normally outputted from the referencevoltage circuit 1004, respectively, a value of contribution of the noisevoltages Vn1, Vn2 to a voltage signal outputted from the operationalamplifier 31 is expressed by the following Equation (1):Vn1{(Cs+Cf)/Cf}×(−R2/R1)+Vn1×(R1+R2)/R1+Vn2×(−Cs/Cf)×(−R2/R1)=Vn1−(R2/R1)×(Cs/Cf)×(Vn1−Vn2)  (1)wherein Cs denotes a capacitance component of the pyroelectric element.R2/R1 is a fairly large number since the voltage amplifying circuit 3has a gain of several tens. Further, if the capacitance component Cs issufficiently large, the noise voltages are amplified.

On the other hand, if a common reference voltage circuit is used as thereference voltage circuits 1003 and 1004, the second term of theEquation (1) is eliminated and only Vn1 remains at the right side of theEquation (1) since Vn1=Vn2. In this aspect, accordingly, it will be seenthat the fifth modification where the reference voltage circuit forapplying the reference voltage to the pyroelectric element 1, thecurrent-to-voltage converting circuit 22 and the voltage amplifyingcircuit 3 is constructed by one circuit, that is, the power supply E,the contribution of the noise voltage of the power supply E to thevoltage signal outputted from the voltage amplifying circuit 3 can bemore suppressed regardless of the gain of the voltage amplifying circuit3 and the capacitance component of the pyroelectric element 1. Further,there is not provided a coupling capacitor in the infrared detectingcircuit. Accordingly, all the parts, i.e., from the current-to-voltageconverting circuit 2 to the output circuit 5, can be integrated into asingle chip, and the influence of external noises is suppressed. Even ifthe reference voltage circuit is constructed by a single power supply E,the external noises is reduced.

FIG. 9 is a circuit diagram of a seventh modified infrared detectingcircuit. The seventh modified infrared detecting circuit is identical tothe sixth modification except for that resistors Ri and R5 areconstructed by switched capacitors SC in a current-to-voltage convertingcircuit 22. Frequency components of around 0.1 to 1.0 Hz are importantin detecting human bodies and living organisms, and the resistors Ri andR5 of the current-to-voltage converting circuit have to be constructedby high resistance members in order to deal with such low-frequencysignals. Since the high resistance member has a large temperaturecharacteristic, the resistance value thereof is caused to largely varyeven by a slight temperature change, thereby hindering a stablecurrent-to-voltage conversion.

Accordingly, in the seventh modified infrared detecting circuit, theabove problem is solved by using the switched capacitors SC having agood temperature characteristic despite their high resistance value asthe resistors Ri, R5 of the current-to-voltage converting circuit 23.

As described above, in the seventh modification, a stablecurrent-to-voltage conversion is possible since the switched capacitorsSC having a good temperature characteristic despite their highresistance value are used as the resistance members of thecurrent-to-voltage converting circuit 23.

FIG. 10 is a circuit diagram of a eighth modified infrared detectingcircuit. The eighth modified infrared detecting circuit is identical tothe infrared detecting circuit shown in FIG. 2 except for that a clockcontrol circuit 9 is provided between a clock generating circuit 6 and aband-pass filter circuit 4, an external clock generator 10 a isconnected with the clock control circuit 9 via an external inputterminal P1, and a controller 10 b is connected with the clock controlcircuit 9 via a clock switching terminal P2.

The clock control circuit 9 selectively switches and outputs a referenceclock signal from the clock generating circuit 6 and a clock signal fromthe external clock generator 10 a to the band-pass filter circuit 4 inaccordance with a clock switching signal inputted from the controller 10b.

The clock generating circuit 6 generates a reference clock signal havinga frequency during a normal operation of the infrared detecting circuitand feeds it to switching elements of the switched capacitors in theinfrared detecting circuit. The external clock generator 10 a generatesa clock signal to be fed to the switching elements of the switchedcapacitors, for example, during a test before shipment. The frequency ofthis clock signal is set to be, for example, 100 times that of the clocksignal generated in the clock generating circuit 6.

This is because the frequency of the reference clock signal generated bythe clock generating circuit 6 is so set as to determine an equivalentresistance value R of the switched capacitors to have a frequencycharacteristic around 1 Hz since frequency components of around 1 Hz areimportant in detecting human bodies, and the switched capacitors areoperated using this clock signal. In other words, since the frequencycharacteristic of the infrared detecting circuit is set to be around 1Hz, at shortest 1 second is required to test such a characteristic,causing a longer time to be required for the test before shipment.

On the other hand, by operating the switched capacitors using the clocksignal having the 100-fold frequency and generated in the external clockgenerator 10 a, the frequency character of the switched capacitors isshifted to around 100 Hz. Thus, the time required to test thecharacteristic can be shortened to 1/100 sec.

Next, the operation of this modification is described. When a clockswitching signal is outputted from the controller 10 b, the clockcontrol circuit 9 switches connection to the external clock generator 10a to feed a clock signal of the external clock generator 10 a to theband-pass filter circuit 4, thereby causing the switched capacitors tooperate in accordance with the external clock signal. When a nextswitching signal is outputted from the controller 10 b, the clockcontrol circuit 9 switches connection to the clock generating circuit 6to feed a reference clock signal of the clock generating circuit 6 tothe band-pass filter circuit 4, thereby causing the switched capacitorsto operate in accordance with the reference clock signal generated inthe clock generating circuit 6, i.e., the clock signal used during thenormal operation.

As described above, in the eighth modification, the clock controlcircuit 9 is provided and the switched capacitors is caused to operatein accordance with the high-frequency clock signal generated in theexternal clock generator 10 a during the test before shipment.Therefore, the time required to test the characteristics can beshortened.

FIG. 11 is a circuit diagram of a part of a ninth modified infrareddetecting circuit, showing a current-to-voltage converting circuit and avoltage amplifying circuit. The ninth modified infrared detectingcircuit is identical to the fifth modification except for that alow-pass filter 80 is connected between the output terminal of anoperational amplifier 221 and the non-inverting input terminal of anoperational amplifier 31.

Upon the application of power, a minute leakage current occurs at theinverting input terminal of the operational amplifier 221. Sinceinverting input terminal of the operational amplifier 221 has aconsiderably high impedance, the leakage current causes the operatingpoint of the voltage signal outputted from the current-to-voltageconverting circuit 22 to largely deviate from the operation point in anormal state. The deviated operating point fluctuates and isconsequently stabilized at the operating point in the normal state. Thisfluctuation of the operating point brings about the saturation of thevoltage amplifying circuit 3. As described earlier, the signalcomponents of around 0.1 to 1.0 Hz are important to detect a human body.The current-to-voltage converting circuit 22 is adapted to convert acurrent signal having this frequency band into a voltage signal by theuse of a capacitor Cf. For this purpose, the resistors Ri and R5 isrequired to have a high resistance. However, it has been known that theintegration of resistors having a high resistance makes the temperaturecharacteristic of the resistors greater, which consequently causing alikelihood that the resistance value rises in the operation. The rise inthe resistance value of the current-to-voltage converting circuit 2shifts the peak frequency of the frequency characteristic toward thehigh frequency side. When the peak frequency is beyond 0.1 Hz, thesignal components in the frequency band of 0.1 to 1.0 Hz is difficult tobe converted into the voltage signal by the capacitor Cf. For thisreason, in consideration of the resistance rise due to the temperaturecharacteristic, the current-to-voltage converting circuit 2 is set tohave a peak frequency considerably lower than 0.1 Hz, e.g., a few mHz.Consequently, a fairly large value is set as a time constant of thecurrent-to-voltage converting circuit 22. This lengthens the fluctuatingperiod of the operating point till stabilization, resulting in a problemthat the voltage amplifying circuit 3 is saturated and does not respondfor a certain period, e.g., for several minutes. This problem is solvedby using the low-pass filter 80 in the ninth modification.

The low-pass filter 80 includes a resistor R6 and a capacitor C4. Theresistor R6 is connected between the output terminal of the operationalamplifier 221 and the non-inverting input terminal of the operationalamplifier 31. The resistor R6 is constructed by an impurity-not-diffusedpolysilicon resistance element, that is, the resistance member is madeof polysilicon in which no impurity is diffused. The capacitor C4 hasone end connected with the non-inverting input terminal of theoperational amplifier 31 and the other end grounded via a power supplyE. The voltage signal outputted from the operational amplifier 221 isbranched into two, one being directly inputted to the inverting inputterminal of the operational amplifier 31 via the resistor R1 and theother being inputted to the non-inverting input terminal of theoperational amplifier 31 via the low-pass filter 80.

The voltage signal passed through the low-pass filter 80 is inputted tothe non-inverting input terminal of the operational amplifier 31 afterhaving frequency components higher than a cutoff frequency removed.Thus, potential at the non-inverting input terminal is not changed bythese high-frequency components.

Since a signal component of the voltage signal outputted from theoperational amplifier 221 and containing frequency components lower thanthe cutoff frequency is inputted at the same phase to the invertinginput terminal and the non-inverting input terminal of the operationalamplifier 31, it is not amplified by the voltage amplifying circuit 3.On the other hand, a signal component of the voltage signal outputtedfrom the operational amplifier 221 and containing frequency componentshigher than the cutoff frequency of the low-pass filter 80 is amplifiedby the voltage amplifying circuit 3 since being inputted only to theinverting input terminal. Accordingly, the voltage amplifying circuit 3is not saturated because the signal component containing frequencycomponents lower than the cutoff frequency, i.e., the signal componentlikely to cause the saturation of the voltage amplifying circuit 3, isnot amplified by the voltage amplifying circuit 3.

The ninth modified infrared detecting circuit in which the low-passfilter 80 is connected with the non-inverting input terminal of theoperational amplifier 31 can reliably prevent the saturation of thevoltage amplifying circuit 3 resulting from the fluctuation of theoperating point of the voltage signal outputted from thecurrent-to-voltage converting circuit 22 during a specified period afterthe application of power.

FIG. 12 is a circuit diagram showing an essential portion of a tenthmodified infrared detecting circuit. This modification is identical tothe ninth modification except for that a low-pass filter 81 is connectedbetween a current-to-voltage converting circuit 22 and a voltageamplifying circuit 3. The low-pass filter 81 is provided with resistorsR8, R9, a switch S81 and a switch controlling circuit 90. The resistorsR8 and R9 are connected between the output terminal of an operationalamplifier 221 and the non-inverting input terminal of an operationalamplifier 31. The switch S81 is connected in parallel with the resistorR8. The respective resistors R8, R9 have resistance values set smallerthan that of the resistor R6 shown in FIG. 11 and impurity-not-diffusedpolysilicon resistance elements are used as such.

The switch controlling circuit 90 controls the switch S81 to keep it onduring a specified period after the application of power and to keep itoff after the lapse of the specified period. The switch controllingcircuit 90 includes a circuit for measuring time, for example, acounter, and starts counting by turning the switch S81 on after theapplication of power. When a count value of this counter reaches apredetermined value set beforehand, the switch S81 is turned off. Forthe integration of the circuit, a semiconductor switching element ispreferably used as the switch S81.

Upon the application of power, the switch controlling circuit 90 turnsthe switch S81 on, thereby short-circuiting the resistor R8. Thus, thetime constant of the low-pass filter 81 is determined by the capacitorC4 and the resistor R9. As a result, the time constant of the low-passfilter 81 is smaller when the switch S81 is on than when it is off,making the cutoff frequency higher.

The impurity-not-diffused polysilicon resistance elements used as theresistors R8, R9 for the integration of the circuit have a fairly largetemperature characteristic, and the cutoff frequency of the low-passfilter 80 shown in FIG. 11 is set at a small value so as to cope with avariation of the resistance value caused by this temperaturecharacteristic. Accordingly, there is a possibility that low frequencysignal components having a low frequency band which brings about thesaturation of the voltage amplifying circuit 3 are much cut during thespecified period after the application of power, and is not introducedto the voltage amplifying circuit 3. For this reason, Accordingly, inthe tenth modification, the saturation of the voltage signal isprevented by using the low-pass filter 81 which enables the timeconstant to decrease during the specified period after the applicationof power.

FIG. 13 is a circuit diagram showing an essential portion of a eleventhmodified infrared detecting circuit. This modification is identical tothe tenth modification except for that a low-pass filter 82 is connectedbetween a current-to-voltage converting circuit 22 and a voltageamplifying circuit 3. The low-pass filter 82 includes a switch S82 and aresistor R10 in addition to those of the low-pass filter 81 of the tenthmodification. The switch S82 is connected with a switch controllingcircuit 91 for controlling the switch S82. The switch S82 and theresistor R10 are connected in series and are connected in parallel withthe resistors R8, R9. Impurity-not-diffused polysilicon resistanceelements are used as the resistors R8, R9, R10. A semiconductorswitching element is used as the switch S82.

The switch controlling circuit 91 controls the switch S82 to turn it onwhen ambient temperature falls to a specified temperature or below,whereby power is applied to the resistor R10 and the time constant ofthe low-pass filter 82 is determined by a combined resistance value ofthe resistors R8, R9, R10 and the capacitance of the capacitor C4. Theresistor R10 is connected in parallel with the resistors R8, R9connected in series. Thus, the combined resistance value of theresistors R8, R9, R10 is smaller than a sum of the resistance values ofthe resistors R8 and R9. Therefore, the time constant of the low-passfilter 82 decreases when the switch S82 is turned on.

The impurity-not-diffused polysilicon resistance elements are used asthe resistors R8, R9 for the integration of the detecting circuit. Sincethe above impurity-not-diffused polysilicon resistors have a property ofincreasing their resistance values as temperature decreases, theresistance values of the resistors R8, R9 increase when ambienttemperature falls, with the result that the time constant of thelow-pass filter 81 increases and the cutoff frequency thereof decreases.Therefore, there is a possibility that voltage signal components in alow-frequency band which brings about the saturation of the voltageamplifying circuit 3 are introduced to the voltage amplifying circuit 3without being sufficiently cut.

Accordingly, in the eleventh modified infrared detecting circuit, thesaturation of the voltage amplifying circuit 3 caused by low temperatureis prevented by using the low-pass filter 82 which enables the timeconstant to decrease when temperature is low.

FIG. 14 is a circuit diagram of the switch S82 and the switchcontrolling circuit 91. The switch S82 includes a switch 821 constructedby a n(negative)-type MOSFET (metal oxide semiconductor field effecttransistor) and a switch 822 constructed by a p(positive)-type MOSFET.The switch controlling circuit 91 includes a controlling circuit 911 forcontrolling the switch 821 and a controlling circuit 912 for controllingthe switch 822.

The controlling circuit 911 is comprised of a resistor R11, switchedcapacitors SC1 and SC2. The resistor R11 is an impurity-not-diffusedpolysilicon resistance element and has one end thereof connected with avoltage supply VCC and the other end thereof connected with avoltage-dividing terminal GP. The switched capacitors SC1, SC2 aresuccessively connected with the voltage-dividing terminal GP. Theswitched capacitor SC1 includes switching elements SC11, SC12 and acapacitor C5, whereas the switched capacitor SC2 includes switchingelements SC12, SC13 and a capacitor C6.

One end of the capacitor C5 is grounded while the other end thereof isconnected between the switching elements SC11 and SC12. Further, one endof the capacitor C6 is grounded while the other end thereof is connectedbetween the switching elements SC12 and SC13.

A clock signal is inputted to the switching elements SC11, SC13 via aterminal CA. A clock signal having a phase opposite from that of theclock signal inputted to the terminal CA is inputted to the switchingelement SC12 via a terminal CB. In this way, the switching elementsSC11, SC12 are alternately turned on and off and the switching elementsSC12 and SC13 are alternately turned on and off. Thus, the switchedcapacitors SC1, SC2 display their function. An equivalent resistance bythe switched capacitor is expressed by R=1/f·C if f denotes a frequencyof the clock signal inputted to the switching element. Since C5=C6=0.5pF and the frequency f of the clock signal to be inputted to theterminals CA, CB is set at 100 Hz (f=100 Hz) in this modification, theequivalent resistance of the switched capacitors SC1, SC2 are 20 GΩ,respectively. The voltage-dividing terminal GP is connected with a gateof the switch 821.

The controlling circuit 912 is comprised of switched capacitors SC3 andSC4 and a resistor R12. One end of the switched capacitor SC3 isconnected with a voltage supply VCC, whereas the other end thereof isconnected with the switched capacitor SC4. The resistor R12 is animpurity-not-diffused polysilicon resistance element and has one endthereof connected between the switched capacitor SC4 and avoltage-dividing terminal GN and the other end thereof grounded. Theswitched capacitor SC3 includes switching elements SC31, SC32 and acapacitor C7, whereas the switched capacitor SC4 includes switchingelements SC32, SC33 and a capacitor C8. A clock signal is inputted tothe switching elements SC31, SC33 via a terminal CA. A clock signalhaving a phase opposite from that of the clock signal inputted to theterminal CA is inputted to the switching element SC32 via a terminal CB.

Similar to the switched capacitors SC1, SC2, the equivalent resistancesof the switched capacitors SC3, SC4 are set at 20 GΩ, respectively.Since the equivalent resistance by the switched capacitor SC1 and theone by the switched capacitor SC2 are connected in series in thecontrolling circuit 911, the equivalent resistances of the switchedcapacitors SC1, SC2 are 40 GΩ. Likewise, the equivalent resistances ofthe switched capacitors SC3, SC4 are 40 GΩ. Thus, if R11=R12=40 GΩ,potentials at the voltage-dividing terminals GP, GN are ½VCC at normalambient temperature. Since the resistance values of the resistors R11,R12 increase as temperature falls, potential at the voltage-dividingterminal GP falls, with the result that potential at thevoltage-dividing terminal GP decreases and potential at thevoltage-dividing terminal GN increases. Therefore, the switches 821, 822are both turned on to apply power to the resistor R10.

As described above, according to the infrared detecting circuit of theeleventh modification, the switch S82 and the resistor R10 are connectedin parallel with the resistors R8, R9 and the switch controlling circuit91 is provided to turn the switch S82 on when temperature is low. Thus,the saturation of the voltage amplifying circuit 3 can be prevented whentemperature is low since a voltage signal component which is in the lowfrequency band and brings about the saturation of the voltage amplifyingcircuit 3 is not amplified. The resistance circuit element Z is used asa circuit for feeding back the direct current component in thecurrent-to-voltage converting circuit. It may be appreciated to use anintegrating circuit as the direct current feedback circuit in place ofthe resistance circuit element Z. In this case, the signal componentsother than the direct current component are fed back while being greatlyattenuated, which consequently effecting more stabilization of theoperating point of the outputted voltage signal.

As described above, a novel infrared detecting circuit, comprises: acurrent-to-voltage converting circuit which is to be connected with apyroelectric element operable to generate a current signal in accordancewith a received infrared ray, and the current-to-voltage convertingcircuit converting the current signal outputted from the pyroelectricelement into a voltage signal, the current-to-voltage converting circuitincluding an operational amplifier connected with the pyroelectricelement, a capacitor, and a feedback circuit for feeding back a directcurrent component, the capacitor and the feedback circuit beingconnected between an output terminal and an inverting input terminal ofthe operational amplifier in parallel with each other; an amplifyingcircuit which amplifies the voltage signal outputted from thecurrent-to-voltage converting circuit; a band-pass filter circuitincluding a switched capacitor and adapted to pass components of avoltage signal from the amplifying circuit in a specified frequencyband; a clock generating circuit which generates a reference clocksignal for controlling the switched capacitor; and an output circuitwhich outputs a voltage signal outputted from the band-pass filtercircuit as a detection signal when the voltage signal is at a thresholdlevel or higher.

In the thus constructed infrared detecting circuit, a detection currentsignal outputted from the pyroelectric element is outputted from theoutput terminal of the operation amplifier after being converted into avoltage signal by the capacitor. The direct current components of theoutputted voltage signal are fed back to the inverting input terminal bythe way of the feedback circuit. Accordingly, the variation of thedirect current components of the outputted voltage signal is suppressed,with the result that an operating point is stabilized. This eliminatesthe need for a coupling capacitor for cutting the variation of theoperating point, thereby enabling the detecting circuit to beminiaturized.

It may be preferable to provide an amplifying circuit between theband-pass filter circuit and the output circuit. With this construction,the voltage signal outputted from the band-pass filter circuit isintroduced to the output circuit after being amplified to a specifiedamplitude level by the second amplifying circuit, thereby setting thethreshold level of the output circuit at a suitable value to increasethe detection accuracy.

It may be preferable to provide a high-pass filter having a specifiedgain between the band-pass filter circuit and the output circuit. Withthis construction, the voltage signal outputted from the band-passfilter circuit has its low-frequency components cut by the high-passfilter. Thus, the variation of the operating point of the band-passfilter circuit is cut. Further, the high-pass filter has the specifiedgain. The voltage signal in the desired frequency band that has thespecified gain is introduced to the subsequent output circuit.Accordingly, the detection accuracy can be remarkably increased.

The band-pass filter circuit may be preferably provided with a high-passfilter at a first stage, a low-pass filter at a second stage, and ahigh-pass filter at a third stage. In other words, the band-pass filtercircuit may be constructed by alternately connecting low-pass filtersand high-pass filters one stage after another. With this construction,the voltage signal outputted from the amplifying circuit has thevariation of the operating point thereof cut by the high-pass filter atthe first stage and has the high-frequency components thereof cut by thelow-pass filter connected at the second or later stage. Then, thevariation of the operating point caused by feedthrough noise during theswitching of the switched capacitor is cut by the high-pass filterconnected at the third or later stage. Accordingly, this constructioncan output a voltage signal having the variation of the operating pointand the return noises cut.

Further, it may be preferable that the low-pass filter has a specifiedgain. With this construction, the band-pass filter circuit can have alarger gain while suppressing the significant variation of the operatingpoint of the low-pass filter. This eliminates the need of providing anamplifying circuit at the subsequent stage of the band-pass filtercircuit to correspond to the detection level of the output circuit,consequently enabling the miniaturization of the detecting circuit.

It may be preferable to provide a high-pass filter having a specifiedgain between the band-pass filter circuit and the output circuit, andthe band-pass filter circuit including a high-pass filter at a firststage, a low-pass filter at a second stage, and a high-pass filter at athird stage. With this construction, the band-pass filter circuitincludes the high-pass filters and the low-pass filters alternatelyconnected with one another, the voltage signal having the variation ofthe operating point due to the feedthrough noises and the return noisescut is sent to the high-pass filter at the following stage. Thehigh-pass filter has the specified gain. Accordingly, the voltage signalin the desired frequency band having the gain is sent to the subsequentoutput circuit. Thus, the detection accuracy can be remarkablyincreased.

The band-pass filter circuit may be preferably provided with a low-passfilter at a first stage, and a high-pass filter at a second stage. Withthis construction, the high-frequency components contained in thevoltage signal outputted from the amplifying circuit are cut in aconcentrated manner by the low-pass filter connected at the first stage,an occurrence of return noise can be suppressed.

The current-to-voltage converting circuit may be preferably providedwith a switched capacitor. With this construction, a high-resistancemember can be equivalently constructed by switching a small capacitor asthe current-to-voltage converting circuit by a clock signal. Thus, thecurrent-to-voltage converting circuit having a small size and a goodtemperature characteristic can be obtained.

Preferably, the infrared detecting circuit may be further provided witha clock control circuit connected between the switched capacitor and theclocking generating circuit and being connectable with an external clockgenerator for generating an external clock signal having a higherfrequency than the reference clock signal of the clock generatingcircuit. The clock control circuit changes over the reference clocksignal and the external clock signal.

With this construction, a clock signal having a frequency different fromthe one during a normal operation can be fed to the switched capacitorfilter of the band-pass filter circuit. For example, the frequencycharacteristic of the circuit shifts toward a higher frequency side byfeeding a clock signal having a frequency higher than the clock signalfed during the normal operation. Since frequency characteristic can bequickly tested by feeding the high-frequency clock signal to theswitched capacitor filter, a testing time can be shortened.

Preferably, the infrared detecting circuit is integrated into a singlesemiconductor chip. Then, the infrared detecting circuit can beminiaturized.

The feedback circuit may be preferably provided with a resistancemember. With this construction, the operating point of the voltagesignal outputted from the operation amplifier can be stabilized by asimple construction.

Preferably, the feedback circuit may be provided with an integratingcircuit. With this construction, the signal components other than thedirect-current component are fed back while being greatly attenuated.The operating point of the outputted voltage signal can be morestabilized.

It may be preferable that the amplifying circuit includes an operationalamplifier, an output terminal of the operational amplifier of thecurrent-to-voltage converting circuit is connected with an invertinginput terminal of the operational amplifier of the amplifying circuitvia a resistance for amplification, and a low-pass filter is connectedbetween the output terminal of the operational amplifier of thecurrent-to-voltage converting circuit and a non-inverting input terminalof the operational amplifier of the amplifying circuit.

With this construction, the voltage signal outputted from the outputterminal of the operational amplifier of the current-to-voltageconverting circuit is branched into two, one being inputted to theinverting input terminal of the operational amplifier of the amplifyingcircuit via the resistance for amplification and the other beinginputted to the non-inverting input terminal of the operationalamplifier of the amplifying circuit after having the high-frequencycomponents removed upon passing the low-pass filter. Accordingly, asignal component of the voltage signal outputted from thecurrent-to-voltage converting circuit containing frequency componentslower than a cutoff frequency of the low-pass filter is inputted at thesame phase to the inverting input terminal and the non-inverting inputterminal of the operational amplifier of the amplifying circuit. Thus,an output from the amplifying circuit is not amplified.

On the other hand, since a signal component containing frequencycomponents higher than the cutoff frequency of the low-pass filter isnot inputted to the non-inverting input terminal of the operationalamplifier of the amplifying circuit, potential at the non-invertinginput terminal does not vary, with the result that this signal componentis outputted after being amplified by the amplifying circuit. Thevoltage signal components in the low frequency band which are likely tocause the amplifying circuit to saturate is not amplified by theamplifying circuit. This prevents the amplifying circuit from beingsaturated by fluctuation of the operating point of the voltage signaloutputted from the current-to-voltage converting circuit due to theinfluence of a leakage current at the inverting input terminal of theoperational amplifier of the current-to-voltage converting circuitduring a specified period after the application of power.

Further, the band-pass filter circuit may be preferably provided with ahigh-pass filter at a first stage, a low-pass filter at a second stage,and a high-pass filter at a third stage, in addition to that theamplifying circuit includes an operational amplifier, an output terminalof the operational amplifier of the current-to-voltage convertingcircuit is connected with an inverting input terminal of the operationalamplifier of the amplifying circuit via a resistance for amplification,and a low-pass filter is connected between the output terminal of theoperational amplifier of the current-to-voltage converting circuit and anon-inverting input terminal of the operational amplifier of theamplifying circuit.

With this construction, the high-pass filter at the first stage cut thevariation of the operating point of the voltage signal outputted fromthe amplifying circuit. The low-pass filters at the second stage cut thehigh frequency components. The high-pass filter at the third stage cutsthe variation of the operating point caused by the feedthrough noisesoccurring when the switched capacitor is operated. Thus, the voltagesignal having a smaller variation of the operating point and withoutreturn noises can be sent to the output circuit.

The voltage signal outputted from the output terminal of the operationalamplifier of the amplifying circuit is branched to the inverting inputterminal of the operational amplifier of the amplifying circuit via theresistance for amplification, and to the non-inverting input terminal ofthe operational amplifier of the amplifying circuit via the low-passfilter while the high frequency component is cut. Accordingly, thevoltage signal component in the frequency band lower than the cutofffrequency of the low-pass filter is input to the inverting andnon-inverting input terminals of the operational amplifier of theamplifying circuit, and is not consequently amplified by the amplifyingcircuit.

On the other hand, the voltage signal in the frequency band higher thanthe cutoff frequency of the low-pass filter is not inputted to thenon-inverting input terminal of the operational amplifier of theamplifying circuit, consequently not changing the potential at thenon-inverting input terminal of the operational amplifier of theamplifying circuit. Accordingly, this signal component is outputtedafter being amplified by the amplifying circuit. The voltage signalcomponents in the low frequency band which are likely to cause theamplifying circuit to saturate is not amplified by the amplifyingcircuit. This prevents the amplifying circuit from being saturated byfluctuation of the operating point of the voltage signal outputted fromthe current-to-voltage converting circuit due to the influence of aleakage current at the inverting input terminal of the operationalamplifier of the current-to-voltage converting circuit during aspecified period after the application of power.

The low-pass filter may be preferably provided with a resistance memberconnected between the output terminal of the operational amplifier ofthe current-to-voltage converting circuit and the non-inverting inputterminal of the operational amplifier of the amplifying circuit, and acapacitor connected between the non-inverting input terminal of theoperational amplifier of the amplifying circuit and the ground. Withthis construction, the low-pass filter can be made in a simplerconstruction.

Further, it may be preferable to provide a switch connected with theresistance member in parallel with each other, and a switch controllerfor controlling the switching circuit.

With this construction, the switch is turned on in accordance with acommand from the switch controller to short-circuit the resistancemember connected in parallel with the switch immediately after theapplication of power. Thus, the time constant of the low-pass filterdecreases and the cutoff frequency thereof increases immediately afterthe application of power. As a result, the voltage signal outputted fromthe current-to-voltage converting circuit is introduced to theamplifying circuit after its signal component in the low frequency bandwhich is likely to cause the amplifying circuit to saturate is securelycut. Therefore, the saturation of the amplifying circuit due to thefluctuation of the operating point during the specified period after theapplication of power can be more securely prevented.

The resistance member may be preferable made of an impurity-not-diffusedpolysilicon. With this construction, the low-pass filter can beintegrated since the resistance member is made of impurity-not-diffusedpolysilicon. Thus, no external part needs to be mounted on the infrareddetecting circuit.

Further, it may be preferable that the low-pass filter is furtherprovided with a secondary resistance circuit connected in parallel withthe resistance member, and a secondary switch controller for turning thesecondary switch on when the ambient temperature is lower than apredetermined value. The secondary resistance circuit has a secondaryresistance member made of an impurity-not-diffused polysilicon, and asecondary switch connected in series with the secondary resistancemember.

With this construction, the resistance value of the secondary resistancemember increases when temperature is low since the secondary resistancemember is constructed by the impurity-not-diffused polysiliconresistance elements. When the resistance value reaches a predeterminedvalue or higher, the secondary switch is turned on by the secondaryswitch controller to apply power to the secondary resistance member. Inother words, since the two resistance members are connected in parallelin the low-pass filter and the time constant of the low-pass filterdecreases when temperature is low, the cutoff frequency of the low-passfilter increases. Thus, when temperature is low, the voltage signal isintroduced to the amplifying circuit after its signal component in thelow frequency which causes the amplifying circuit to saturate issecurely cut. Therefore, the saturation of the amplifying circuit whentemperature is low can be prevented.

Preferably, the secondary switch controller may be provided with aswitched capacitor for producing an equivalent resistance, therebycontrolling the switching circuit using a voltage divided by anequivalent resistance produced by the switched capacitor and resistancesof the resistance member.

With this construction, when ambient temperature is low, the resistancesof the impurity-not-diffused polysilicon resistance members increase andthe voltage divided by the resistances acts to turn the secondary switchon, thereby turning the secondary switch on. In other words, sincechanges in the resistance values of the impurity diffuse polysiliconresistance members are detected using the impurity diffuse polysiliconresistance member, changes in the resistance values caused by thetemperature characteristic of the impurity diffuse polysiliconresistance members can be precisely detected. In addition, since the tworesistance members are made of impurity diffuse polysilicon, thedetecting circuit can be integrated.

Preferably, the infrared detecting circuit may be further provided witha reference voltage circuit for generating a reference voltage, thereference voltage circuit being connected with the pyroelectric elementand the non-inverting input terminals of the respective operationalamplifiers of the current-to-voltage converting circuit and theamplifying circuit.

With this construction, the reference voltage to be fed to thepyroelectric element and the one to be fed to the non-inverting inputterminals of the two operational amplifiers are given by the singlereference voltage circuit. This sets off amplification of noises whichare likely to influence the output of the amplifying circuit, andreduces the occurrence of noises of the infrared detecting circuit.

A novel infrared detector comprises the above-mentioned infrareddetecting circuit, and a pyroelectric element for receiving an infraredray and produces a current signal in accordance with the receivedinfrared ray. With this construction, an infrared detector can beminiaturized since having the miniaturized infrared detecting circuit.

As this invention may be embodied in several forms without departingfrom the spirit of essential characteristics thereof, the presentexamples are therefore illustrative and not restrictive, since the scopeof the invention is defined by the appended claims rather than by thedescription preceding them, and all changes that fall within metes andbounds of the claims, or equivalence of such metes and bounds aretherefore intended to embraced by the claims.

INDUSTRIAL APPLICABILITY

A novel infrared detecting circuit and infrared detector can beminiaturized by directly connecting a current-to-voltage convertingcircuit and a voltage amplifying circuit. The variation of the operating{circumflex over (p)}oint can be considerably cut to increase thedetection accuracy remarkably. A novel infrared detecting circuit andnovel infrared detector which are smaller in the size and have a higherdetection accuracy are widely usable in the infrared detection field.

1. An infrared detecting circuit, comprising: a current-to-voltageconverting circuit that is connectable to a pyroelectric elementoperable to generate a current signal in accordance with a receivedinfrared ray, wherein the current-to-voltage converting circuit convertsthe current signal output from the pyroelectric element into a voltagesignal, the current-to-voltage converting circuit includes anoperational amplifier, a capacitor, and a feedback circuit for feedingback a direct current component, the capacitor and the feedback circuitbeing connected between an output terminal and an inverting inputterminal of the operational amplifier in parallel with each other; anamplifying circuit that amplifies the voltage signal output from thecurrent-to-voltage converting circuit; a band-pass filter circuit thatis configured to pass components of a voltage signal from the amplifyingcircuit in a specified frequency band, wherein the band-pass filtercircuit includes a switched capacitor; a clock generating circuit whichgenerates a reference clock signal for controlling the switchedcapacitor; and an output circuit that outputs a voltage signal outputfrom the band-pass filter circuit as a detection signal when the voltagesignal is at a threshold level or higher, wherein a high-pass filterhaving a specified gain is connected between the band-pass filtercircuit and the output circuit.
 2. An infrared detecting circuitaccording to claim 1, wherein an additional amplifying circuit isconnected between the band-pass filter circuit and the output circuit.3. An infrared detecting circuit according to claim 1, wherein theband-pass filter circuit includes a high-pass filter at a first stage, alow-pass filter at a second stage, and a high-pass filter at a thirdstage.
 4. An infrared detecting circuit according to claim 1, whereinthe band-pass filter circuit includes a low-pass filter at a firststage, and a high-pass filter at a second stage.
 5. An infrareddetecting circuit according to claim 1, wherein the current-to-voltageconverting circuit, the amplifying circuit, the band-pass filtercircuit, and the output circuit are integrated into a single chip.
 6. Aninfrared detecting circuit according to claim 1, wherein the feedbackcircuit includes a resistance member.
 7. An infrared detecting circuitaccording to claim 1, wherein the feedback circuit includes anintegrating circuit.
 8. An infrared detecting circuit, comprising: acurrent-to-voltage converting circuit that is connectable to apyroelectric element operable to generate a current signal in accordancewith a received infrared ray, wherein the current-to-voltage convertingcircuit converts the current signal output from the pyroelectric elementinto a voltage signal, the current-to-voltage converting circuitincludes an operational amplifier, a capacitor, and a feedback circuitfor feeding back a direct current component, the capacitor and thefeedback circuit being connected between an output terminal and aninverting input terminal of the operational amplifier in parallel witheach other; an amplifying circuit that amplifies the voltage signaloutput from the current-to-voltage converting circuit; a band-passfilter circuit that is configured to pass components of a voltage signalfrom the amplifying circuit in a specified frequency band, wherein theband-pass filter circuit includes a switched capacitor; a clockgenerating circuit which generates a reference clock signal forcontrolling the switched capacitor; and an output circuit that outputs avoltage signal output from the band-pass filter circuit as a detectionsignal when the voltage signal is at a threshold level or higher,wherein the band-pass filter circuit includes a high-pass filter at afirst stage, a low-pass filter at a second stage, and a high-pass filterat a third stage.
 9. An infrared detecting circuit according to claim 8,wherein the low-pass filter has a specified gain.
 10. An infrareddetecting circuit, comprising: a current-to-voltage converting circuitthat is connectable to a pyroelectric element operable to generate acurrent signal in accordance with a received infrared ray, wherein thecurrent-to-voltage converting circuit converts the current signal outputfrom the pyroelectric element into a voltage signal, thecurrent-to-voltage converting circuit includes an operational amplifier,a capacitor, and a feedback circuit for feeding back a direct currentcomponent, the capacitor and the feedback circuit being connectedbetween an output terminal and an inverting input terminal of theoperational amplifier in parallel with each other; an amplifying circuitthat amplifies the voltage signal output from the current-to-voltageconverting circuit; a band-pass filter circuit that is configured topass components of a voltage signal from the amplifying circuit in aspecified frequency band, wherein the band-pass filter circuit includesa switched capacitor; a clock generating circuit which generates areference clock signal for controlling the switched capacitor; and anoutput circuit that outputs a voltage signal output from the band-passfilter circuit as a detection signal when the voltage signal is at athreshold level or higher, wherein the current-to-voltage convertingcircuit includes a switched capacitor.
 11. An infrared detectingcircuit, comprising: a current-to-voltage converting circuit that isconnectable to a pyroelectric element operable to generate a currentsignal in accordance with a received infrared ray, wherein thecurrent-to-voltage converting circuit converts the current signal outputfrom the pyroelectric element into a voltage signal, thecurrent-to-voltage converting circuit includes an operational amplifier,a capacitor, and a feedback circuit for feeding back a direct currentcomponent, the capacitor and the feedback circuit being connectedbetween an output terminal and an inverting input terminal of theoperational amplifier in parallel with each other; an amplifying circuitthat amplifies the voltage signal output from the current-to-voltageconverting circuit; a band-pass filter circuit that is configured topass components of a voltage signal from the amplifying circuit in aspecified frequency band, wherein the band-pass filter circuit includesa switched capacitor; a clock generating circuit which generates areference clock signal for controlling the switched capacitor; an outputcircuit that outputs a voltage signal output from the band-pass filtercircuit as a detection signal when the voltage signal is at a thresholdlevel or higher; and a clock control circuit which is connected betweenthe switched capacitor and the clock generating circuit, and isconnectable with an external clock generator for generating an externalclock signal having a higher frequency than the reference clock signalof the clock generating circuit, whereby the clock control circuitchanges over the reference clock signal and the external clock signal.12. An infrared detecting circuit, comprising: a current-to-voltageconverting circuit that is connectable to a pyroelectric elementoperable to generate a current signal in accordance with a receivedinfrared ray, wherein the current-to-voltage converting circuit convertsthe current signal output from the pyroelectric element into a voltagesignal, the current-to-voltage converting circuit includes anoperational amplifier, a capacitor, and a feedback circuit for feedingback a direct current component, the capacitor and the feedback circuitbeing connected between an output terminal and an inverting inputterminal of the operational amplifier in parallel with each other; anamplifying circuit that amplifies the voltage signal output from thecurrent-to-voltage converting circuit; a band-pass filter circuit thatis configured to pass components of a voltage signal from the amplifyingcircuit in a specified frequency band, wherein the band-pass filtercircuit includes a switched capacitor; a clock generating circuit whichgenerates a reference clock signal for controlling the switchedcapacitor; and an output circuit that outputs a voltage signal outputfrom the band-pass filter circuit as a detection signal when the voltagesignal is at a threshold level or higher, wherein the amplifying circuitincludes an operational amplifier, an output terminal of the operationalamplifier of the current-to-voltage converting circuit is connected withan inverting input terminal of the operational amplifier of theamplifying circuit via a resistance for amplification, and a low-passfilter is connected between the output terminal of the operationalamplifier of the current-to-voltage converting circuit and anon-inverting input terminal of the operational amplifier of theamplifying circuit.
 13. An infrared detecting circuit according to claim12, wherein the band-pass filter circuit includes a high-pass filter ata first stage, a low-pass filter at a second stage, and a high-passfilter at a third stage.
 14. An infrared detecting circuit according toclaim 12, wherein the low-pass filter includes: a resistance memberconnected between the output terminal of the operational amplifier ofthe current-to-voltage converting circuit and the non-inverting inputterminal of the operational amplifier of the amplifying circuit; and acapacitor connected between the non-inverting input terminal of theoperational amplifier of the amplifying circuit and the ground.
 15. Aninfrared detecting circuit according to claim 14, further comprising: aswitch connected with the resistance member in parallel with each other;and a switch controller which controls the switch.
 16. An infrareddetecting circuit according to claim 15, wherein the resistance membercomprises an impurity-not-diffused polysilicon.
 17. An infrareddetecting circuit according to claim 16, wherein the low-pass filterfurther includes: a secondary resistance circuit connected in parallelwith the resistance member, and comprising: a secondary resistancemember comprising an impurity-not-diffused polysilicon; and a secondaryswitch connected in series with the secondary resistance member; asecondary switch controller which turns the secondary switch on when theambient temperature is lower than a predetermined value.
 18. An infrareddetecting circuit according to claim 17, wherein the secondary switchcontroller includes a switched capacitor which produces an equivalentresistance, thereby controlling the switch using a voltage divided by anequivalent resistance produced by the switched capacitor and resistancesof the resistance member.
 19. An infrared detecting circuit, comprising:a current-to-voltage converting circuit that is connectable to apyroelectric element operable to generate a current signal in accordancewith a received infrared ray, wherein the current-to-voltage convertingcircuit converts the current signal output from the pyroelectric elementinto a voltage signal, the current-to-voltage converting circuitincludes an operational amplifier, a capacitor, and a feedback circuitfor feeding back a direct current component, the capacitor and thefeedback circuit being connected between an output terminal and aninverting input terminal of the operational amplifier in parallel witheach other; an amplifying circuit that amplifies the voltage signaloutput from the current-to-voltage converting circuit; a band-passfilter circuit that is configured to pass components of a voltage signalfrom the amplifying circuit in a specified frequency band, wherein theband-pass filter circuit includes a switched capacitor; a clockgenerating circuit which generates a reference clock signal forcontrolling the switched capacitor; an output circuit that outputs avoltage signal output from the band-pass filter circuit as a detectionsignal when the voltage signal is at a threshold level or higher; and areference voltage circuit which generates a reference voltage, thereference voltage circuit being connected with the pyroelectric elementand the non-inverting input terminals of the respective operationalamplifiers of the current-to-voltage converting circuit and theamplifying circuit.